Your Ad HereNapa County is a county located in the U.S. state of California. As of 2016 the population is 142,166. The county seat is Napa.HistoryNapa County was formed and became one of the original California counties when the state became part of the United States in 1849. The word napa is of Native American derivation. Of the many explanations of the name's origin, the most plausible seems to be that it is derived from the Patwin word napo meaning house, although local residents will often cite an urban legend that gives the translation as "you will always return".